1. Field of the Invention
This invention broadly relates to positioning of spacecraft and more particularly to attitude control.
2. Description of the Prior Art
Attitude control of spin stabilized spacecraft has traditionally been accomplished by pulsed thrusters which provide spin phased moments. This works well when the operational spin axis is normal to the orbit plane. The payloads in some spacecraft, however, require a nadir pointing spin axis. In order to preclude continuous thrusting to maintain the orbital angular rate, momentum cancelling wheels hard mounted to the spacecraft are used to create a zero momentum system. When it is desired to slew the spin axis from one pointing direction to another, reaction control thrusters are used to create the starting and stopping moments. If solar radiation pressure creates undesired external moments, thrusters are also used to cancel such moments.
While reaction control thrusters are necessary, it would be desirable to use them as little as possible. There is a limited amount of propellant available to fire the thrusters. The more that the thrusters must be fired, the less fuel available for later maneuvers. Further, there are inherent uncertainties involving a thruster on-time and sophisticated methods are required in order to compensate for these uncertainties.